Inverter generator

ABSTRACT

In an inverter generator having an engine generator unit, a converter that converts generated alternating current to direct current, an inverter that converts the direct current to alternating current with switching elements to supply to an electrical load, an inverter driver that drives the switching elements with a PWM signal and makes the alternating current of a predetermined frequency, the alternating current supplied to the electrical and voltages of the direct and alternating currents are detected, the detected voltage of the alternating current is corrected as a predetermined value based on a coefficient (DCgainA) set based on the detected voltage of the direct current, when the detected alternating current is greater than a threshold value, and the PWM signal is corrected by the predetermined, thereby limiting overcurrent.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an inverter generator, particularly to aninverter generator equipped with a generator unit driven by an internalcombustion engine and adapted to limit overcurrent.

2. Description of the Related Art

One well-known inverter generator once converts the alternating currentoutputted by an engine-driven generator unit to direct current and thenconverts the direct current into alternating current of a predeterminedfrequency (utility frequency) by driving switching elements with a PWMsignal generated using a reference sine wave of the desired outputvoltage waveform and a carrier. An example of such an inverter generatorcan be found in Japanese Laid-Open Patent Application No.H4(1992)-355672.

In such the inverter generator taught by the reference, an overcurrentlimiter circuit is provided to protect the switching elements fromovercurrent caused by short-circuit or inrush load. When the detectedcurrent exceeds the tolerance limit, the circuit makes a PWM signalsupplied to the switching elements zero to drop the output current zerotemporarily.

SUMMARY OF THE INVENTION

The overcurrent can thus be prevented once by the overcurrent limitercircuit. Since, however, the output current is made zero, the PWM signalis again supplied so that the current again exceeds the tolerance limit,then the PWM signal is again made zero so that the output current ismade zero temporarily, and it goes on. It is disadvantageous that aseries of the same events is repeated. Further, since the tolerancelimit is set to a relatively high value, it is preferable to limit theovercurrent at a level lower than the set limit value.

This invention is therefore directed to overcoming the aforesaid problemby providing an inverter generator that conducts conversion toalternating current of a predetermined frequency based on a PWM signalgenerated using a reference sine wave of the desired output voltagewaveform and a carrier, wherein overcurrent can be reliably limited orrestricted.

In order to achieve the object, this invention provides in its firstaspect an inverter generator having a generator unit that is driven byan internal combustion engine and generates alternating current, aconverter that is connected to the generator unit and converts thealternating current to direct current, an inverter that is connected tothe converter and converts the direct current to alternating currentwith switching elements to supply to an electrical load, an inverterdriver that drives the switching elements with a PWM signal generatedusing a reference sine wave of a desired output voltage waveform and acarrier at every control cycle and makes the alternating currentconverted in the inverter to the alternating current of a predeterminedfrequency, comprising: a current detector that detects the alternatingcurrent supplied to the electrical load; a direct current voltagedetector that detects voltage of the direct current converted by theconverter; an alternating current voltage detector that detects voltageof the alternating current supplied by the inverter; an output voltagecorrector that corrects the detected voltage of the alternating currentas a predetermined value based on a coefficient set based on thedetected voltage of the direct current, when the detected alternatingcurrent is greater than a threshold value; and a PWM signal correctorthat corrects the PWM signal by the predetermined value at the controlcycle, such that the detected alternating current becomes less than thethreshold value.

In order to achieve the object, this invention provides in its secondaspect a method of controlling an inverter generator having a generatorunit that is driven by an internal combustion engine and generatesalternating current, a converter that is connected to the generator unitand converts the alternating current to direct current, an inverter thatis connected to the converter and converts the direct current toalternating current with switching elements to supply to an electricalload, an inverter driver that drives the switching elements with a PWMsignal generated using a reference sine wave of a desired output voltagewaveform and a carrier at every control cycle and makes the alternatingcurrent converted in the inverter to the alternating current of apredetermined frequency, comprising the steps of: detecting thealternating current supplied to the electrical load; detecting voltageof the direct current converted by the converter; detecting voltage ofthe alternating current supplied by the inverter; correcting thedetected voltage of the alternating current as a predetermined valuebased on a coefficient set based on the detected voltage of the directcurrent, when the detected alternating current is greater than athreshold value; and correcting the PWM signal by the predeterminedvalue at the control cycle, such that the detected alternating currentbecomes less than the threshold value.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the invention will be moreapparent from the following description and drawings in which:

FIG. 1 is a block diagram giving an overview of an inverter generatoraccording to an embodiment of this invention;

FIG. 2 is a waveform diagram for explaining a PWM control by a CPU shownin FIG. 1;

FIG. 3 is a flowchart showing the operation of the CPU shown in FIG. 1;

FIG. 4 is a waveform diagram showing an AC voltage waveform outputtedfrom an inverter shown in FIG. 1; and

FIG. 5 is a time chart for explaining the processing in the flowchart ofFIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An inverter generator according to an embodiment of this invention willnow be explained with reference to the attached drawings.

FIG. 1 is a block diagram giving an overview of an inverter generatoraccording to an embodiment of this invention.

The inverter generator is designated by reference numeral 10 in FIG. 1.The generator 10 is equipped with an engine (internal combustion engine)12 and has a rated output of about 3 kW (AC 100 V, 30 A). The engine 12is an air-cooled, spark-ignition engine. Its throttle valve 12 a isopened and closed by a throttle motor (actuator) 12 b constituted as astepper motor. The engine 12 is started with a recoil starter (notshown).

A circular stator (not shown) is fastened near the cylinder head of theengine 12. The stator is provided with windings that constitute anengine generator unit 14, namely with three-phase (U, V and W) outputwindings (main windings) 14 a and three single-phase windings 14 b, 14 cand 14 d.

A rotor (not shown) that doubles as the flywheel of the engine 12 isinstalled in the outside of the stator. Permanent magnets (not shown)are attached in the rotor at positions opposite the aforesaid windings14 a etc. and with their radially oriented polarities reversedalternately.

When the permanent magnets of the rotor surrounding the stator rotate,three-phase (U, V and W phase) alternating current is outputted from(generated by) the three-phase output windings 14 a and single-phasealternating current is outputted from the single-phase output windings14 b, 14 c and 14 d.

The three-phase alternating current outputted from (generated by) theoutput windings 14 a of the generator unit 14 is passed through U, V andW terminals 14 e to a control board (printed board) 16 and inputted to aconverter 20 mounted thereon. The converter 20 is equipped withbridge-connected three thyristors (SCRs) and three diodes DI. Thethree-phase alternating current outputted by the generator unit 14 isconverted to direct current by controlling the conduction angles of thethyristors.

A ringing choke converter (RCC) power supply (direct current stabilizedpower supply) 22 is connected to the positive and negative electrodeside outputs of the converter 20 and supplies the rectified DC power tothe three thyristors as operating power. A smoothing capacitor 24 isconnected downstream of the RCC power supply 22 to smooth the directcurrent outputted from the converter 20.

An inverter 26 is connected downstream of the smoothing capacitor 24.The inverter 26 is equipped with a four-FET bridge circuit (FET: fieldeffect transistor (switching element)). As explained further below, thedirect current outputted from the converter 20 is converted toalternating current of a predetermined frequency (50 Hz or 60 Hz utilitypower frequency) by controlling the conducting (ON-OFF) state of thefour FETs.

The output of the inverter 26 is passed through a choke coil 30 composedof an LC filter for harmonic suppression and through a noise filter 32for noise suppression to be applied to output terminals 34, from whichit can be supplied to an electrical load 36 through a connector (notshown) or the like.

The control board 16 is equipped with a CPU (central processing unit) 40having a 32-bit architecture. The CPU 40 controls the conduction angleof the thyristors of the converter 20 though a thyristor (SCR) driver(drive circuit) 40 a, the conducting state of the FETs of the inverter26 through a gate driver 40 b, and the operation of the throttle motor12 b through a motor driver 40 c. The CPU 40 is equipped with an EEPROM(nonvolatile memory) 40 d.

The output of the first single-phase output winding 14 b is sent to thecontrol board 16 through sub-terminals 14 b 1 and 14 b 2, where it isinputted to a control power generator 14 b 3 that generates 5 Voperating power for the CPU 40. The output from the sub-terminal 14 b 1is sent to an NE detection circuit 14 b 4, where it is converted to apulse signal and sent to the CPU 40. The CPU 40 counts the pulses of theoutput from the NE detection circuit 14 b 4 to calculate (detect) thespeed NE of the engine 12.

The output of the second output winding 14 c is sent to a full-waverectifier circuit 14 c 1, where it is full-wave rectified to produceoperating power for, inter alia, the throttle motor 12 b. The output ofthe third output winding 14 d is sent to an ignition circuit 12 c of theengine 12 for use as ignition power for a spark plug 12 d.

The CPU 40 is connected to first and second voltage sensors (detectors)40 e and 40 f. The first voltage sensor 40 e on downstream of the RCCpower supply 22 produces an output or signal proportional to the DCvoltage output of the converter 20. The second voltage sensor 40 f ondownstream of the inverter 26 produces an output or signal proportionalto the AC voltage output of the inverter 26. The outputs of the firstand second voltage sensors 40 e and 40 f are sent to the CPU 40.

The CPU 40 is further connected to a current sensor (detector) 40 g. Thecurrent sensor 40 g produces an output or signal proportional to thecurrent outputted from the inverter 26, i.e., the current passingthrough the electrical load 36 when the load 36 is connected.

The output of the current sensor 40 g is inputted to the CPU 40 and alsoto an overcurrent limiter 40 h constituted as a logic circuit (hardwarecircuit) independent of the CPU 40. When the current detected by thecurrent sensor 40 g exceeds a tolerance limit, the overcurrent limiter40 h suspends the output of the gate driver 40 b to make the output ofthe inverter 26 zero temporarily.

The CPU 40 is inputted with the outputs of the first and second voltagesensors 40 e, 40 f and current sensor 40 g and based thereon,PWM-controls the FETs of the inverter 26, controls the operation of thethrottle motor 12 b, and further controls overcurrent limiting.

FIG. 2 is a waveform diagram for explaining the PWM control by the CPU40.

Explaining the PWM control on the FETs of the inverter 26 with referenceto FIG. 2, based on a reference sine wave (signal wave) with respect tothe predetermined frequency (50 Hz or 60 Hz utility power frequency) ofthe desired AC output voltage waveform (lower broken-line wave), the CPU40 uses a comparator (not shown) to compare it with a carrier (e.g., a20 kHz carrier wave), produces a PWM signal (PWM waveform), namely avariable duty ratio (=ON time t/period T) pulse train, in accordancewith PWM (pulse width modulation), and outputs the signal through thegate driver 40 b.

The lower broken-line wave in FIG. 2 indicates the desired outputvoltage waveform. The period T (step) of the PWM signal (PWM waveform),which is actually much shorter than shown, is enlarged in FIG. 2 forease of understanding.

The CPU 40 determines the opening of the throttle valve 12 a toestablish the desired engine speed calculated based on the AC outputconsumed by the electrical load 36, calculates A phase and B phaseoutput pulses for the throttle stepper motor 12 b, and supplies themthrough the motor driver 40 c to the throttle stepper motor 12 b fromoutput terminals 40 c 1, thereby controlling the operation of thethrottle motor 12 b.

Next, among the control operations of the CPU 40, the operation ofovercurrent limiting control will be explained.

FIG. 3 is a flowchart showing the operation.

The illustrated program is executed at every predetermined controlcycle, for example every 50 microseconds in the case where the frequencyof the carrier shown in FIG. 2 is 20 kHz and the frequency of the outputvoltage waveform is 50 Hz. More specifically, it is executed every stepin the graph of FIG. 2.

Explaining in the following, the program begins in S10, in which it isdetermined whether control starting conditions are met. The controlstarting conditions are that a premise condition is established, that anabsolute value of A/D converted value (effective value) of the currentdetected by the current sensor 40 g is greater than a peak current limitvalue (threshold value), and that the bit of apeak-current-limiting-execution flag (explained later) was OFF in thepreceding program execution of the FIG. 3 flowchart.

The premise condition is a power factor being equal to or greater than0.9. The peak current limit value is set to be lower than the tolerancelimit used by the overcurrent limiter 40 h.

FIG. 4 is a waveform diagram showing an AC voltage waveform outputtedfrom the inverter 26 and FIG. 5 is a time chart for explaining thecontrol operation in the flowchart of FIG. 3.

As shown in FIGS. 4 and 5, in the control operation, if the currentexceeds the peak current limit value, the voltage (corresponding to thecurrent) is controlled to reduce the current to a value below the peakcurrent limit value. Accordingly, when the phase difference between thecurrent and voltage is large, in other words the force factor is small,it becomes difficult to determine the correspondence of the current andvoltage. For that reason, the power factor being equal to or greaterthan 0.9 is included as one of the control starting conditions.

In addition, since the peak current limit value is set on both of thepositive and negative sides, the A/D converted value of the detectedcurrent is compared with the limit value in terms of the absolute value.

The explanation of FIG. 3 flowchart is resumed.

When the result in S10 is Yes, the program proceeds to S12, in which thebit of the peak-current-limiting-execution flag is made ON, i.e., set to1, and to S14, in which the output voltage amplitude value in thepreceding program execution, i.e., the output voltage amplitude value inthe preceding control cycle is read and renamed (and stored) as a peakcurrent limit amplitude value, and a DC voltage A/D value in the presentprogram execution, i.e., a DC voltage A/D value in the present controlcycle is read and renamed (and stored) as a peak current limit DCvoltage value. The DC voltage is the voltage of direct current outputtedfrom the converter 20.

The program proceeds to S16, in which it is determined whether the bitof the peak-current-limiting-execution flag is ON and when the result isNo, the remaining steps are skipped. When the result is Yes, the programproceeds to S18, in which it is determined whether the output voltageamplitude value at the preceding control cycle is equal to or greaterthan zero and whether the output voltage amplitude value at the presentcontrol cycle is equal to or greater than that in the preceding controlcycle, i.e., it is determined whether it is in the rising stage on thepositive side in the graph of FIG. 4.

When the result in S18 is No, the remaining steps are skipped and whenthe result is Yes, the program proceeds to S20, in which the peakcurrent limit DC voltage value stored in S14 is divided by the DCvoltage A/D value at the present control cycle and the obtained quotientis determined as a DC voltage fluctuation coefficient DCgainA at thepresent control cycle.

Thus, the DC voltage fluctuation coefficient DCgainA means a quotientobtained by dividing the peak current limit DC voltage value by the DCvoltage A/D value at the present control cycle. Since the peak currentlimit DC voltage value is also the DC voltage A/D value at the presentcontrol cycle renamed and stored in S14, the DC voltage fluctuationcoefficient DCgainA will be a coefficient indicating the fluctuationrate of the DC voltage.

The program then proceeds to S22, in which the peak current limitamplitude value is multiplied by the DC voltage fluctuation coefficientDCgainA determined or calculated in S20 and the obtained product isdetermined as the output voltage amplitude value at the present controlcycle.

Since the peak current limit amplitude value is the output voltageamplitude value at the preceding control cycle renamed and stored inS14, the above processing amounts to determining a value obtained bymultiplying the output voltage amplitude value at the preceding controlcycle by the DC voltage fluctuation coefficient DCgainA as the outputvoltage amplitude value at the present control cycle.

The program then proceeds to S24, in which it is determined whether theoutput voltage amplitude value at the preceding control cycle is lessthan zero and whether the output voltage amplitude value at the presentcontrol cycle is less than that in the preceding control cycle, i.e., itis determined whether it is in the rising stage on the negative side inthe graph of FIG. 4.

When the result is No, the remaining steps are skipped and when theresult is Yes, the program proceeds to S26, in which, similarly to theforegoing processing, a product (negative value) obtained by multiplyingthe peak current limit amplitude value by the DC voltage fluctuationcoefficient DCgainA is determined as the output voltage amplitude valueat the present control cycle.

The program next proceeds to S28, in which the PWM signal is correctedbased on the output voltage amplitude value at the present controlcycle. Specifically, the duty ratio in the graph of FIG. 2 is decreasedto make the output voltage waveform trapezoidal as shown in FIG. 5.

When the result in S10 is No, the program proceeds to S30, in which itis determined whether the premise condition (the power factor is equalto or greater than 0.9) is not established or whether the absolute valueof the current A/D converted value (effective value) is less than a peakcurrent limit restoration value. When the result is No, the remainingsteps are skipped and when the result is Yes, the program proceeds toS32, in which the aforementioned flag is made OFF, i.e., the bit thereofis reset to zero and the program is terminated.

The operation of FIG. 3 flowchart will be explained with reference toFIG. 5.

As described above, the inverter generator 10 is unable to limit currentand hence can only limit voltage to limit the overcurrent. In view ofthis, in the embodiment, the generator 10 is configured to limit thevoltage to a value at the time when the current exceeds the peak currentlimit value, i.e., to the peak current limit amplitude value at thatcontrol cycle (program execution).

Further, the inverter 26 of the generator 10 can not output alternatingcurrent greater in voltage than direct current outputted from theconverter 20. In addition, as explained above, the operation of thethrottle motor 12 b is controlled in accordance with the AC outputdetermined by the electrical load 36. At any rate, the voltage in directcurrent or alternating current must fluctuate in terms of instantaneousvalue.

This embodiment is therefore configured to obtain the DC voltagefluctuation coefficient and multiply the limit value (peak current limitamplitude value) by the coefficient. Owing to this configuration, whenthe current is about to exceed the peak current limit value, the outputvoltage can be limited to a certain constant value regardless offluctuation in the electrical load 36, as shown in FIG. 5.

When the current becomes below the peak current limit restoration value,the limiting operation is canceled and the output voltage waveform isrestored or returned to the sine waveform as shown in FIG. 2. The peakcurrent limit restoration value is set in the vicinity of the peakcurrent limit value for avoiding control hunting.

As stated above, the embodiment is configured to have an invertergenerator (10) (and a method of controlling the inverter generator (10))having a generator unit (14) that is driven by an internal combustionengine (12) and generates alternating current, a converter (20) that isconnected to the generator unit and converts the alternating current todirect current, an inverter (26) that is connected to the converter andconverts the direct current to alternating current with switchingelements to supply to an electrical load (36), an inverter driver (CPU40) that drives the switching elements with a PWM signal generated usinga reference sine wave of a desired output voltage waveform and a carrierat every control cycle and makes the alternating current converted inthe inverter to the alternating current of a predetermined frequency,characterized by: a current detector (CPU 40, 40 g, S10) that detectsthe alternating current supplied to the electrical load (36); a directcurrent voltage detector (CPU 40, 40 e) that detects voltage of thedirect current converted by the converter; an alternating currentvoltage detector (CPU 40, 40 f) that detects voltage of the alternatingcurrent supplied by the inverter; an output voltage corrector (CPU 40,S10 to S26) that corrects the detected voltage of the alternatingcurrent (output voltage amplitude value) as a predetermined value(output voltage amplitude value) based on a coefficient (DC voltagefluctuation coefficient DCgainA) set based on the detected voltage ofthe direct current, when the detected alternating current is greaterthan a threshold value (peak current limit value); and a PWM signalcorrector (CPU 40, S28) that corrects the PWM signal by thepredetermined value at the control cycle, such that the detectedalternating current becomes less than the threshold value.

Thus, it is configured to detect the current supplied to the load 36,the voltage of direct current outputted from the converter 20 and thevoltage of alternating current outputted from the inverter 26, correctthe detected DC voltage to a predetermined value based on a value set inaccordance with the detected DC voltage when the detected currentexceeds the threshold value (peak current limit value), correct the PWMsignal used for operating the switching element in every control cyclebased on the corrected value, thereby decreasing the current to a valuebelow the threshold value. With this, since the AC output voltage can beheld at the predetermined value when the current exceeds the thresholdvalue, it becomes possible to limit the current at a constant levelbelow the threshold value, thereby reliably limiting overcurrent.

More specifically, although the inverter generator 10 can not directlycontrol the current due to its attributes, it controls the voltageinstead of the current, thereby decreasing the current to a value belowthe threshold value. Further, since the voltage is controlled based onthe inputted DC voltage, it becomes possible to reliably decrease thecurrent to a value below the threshold value regardless of fluctuationin the electrical load 36.

In the generator, the coefficient (DCgainA) is set based on the detectedvoltages at different control cycles, specifically the coefficient isset based on a ratio of the detected voltages at different controlcycles, more specifically the coefficient is set based on a ratio of thedetected voltages at preceding control cycle and present control cycle(S10).

In the generator, the output voltage corrector corrects the detectedvoltage of the alternating current as the predetermined value when apower factor is equal to or greater than a prescribed value (S10).

In the generator, the current detector detects the alternating currentas an effective value based on an detected value obtained by a currentsensor (40 g).

It should be noted that, in the embodiment, the term of the “precedingcontrol cycle” is not limited to a value one cycle before but can be avalue two or more cycles before or an average of values in multiplecycles.

Although a ratio is used to obtain the coefficient (DC voltagefluctuation coefficient DCgainA) set based on the voltage of directcurrent detected in the FIG. 3 flowchart, a difference can be utilizedinstead.

Although FETs are used as the switching elements of the inverter in theforegoing, this is not a limitation and it is possible to use insulatedgate bipolar transistors (IGBTs) or the like instead.

Japanese Patent Application No. 2008-191780 filed on Jul. 25, 2008, isincorporated herein in its entirety.

While the invention has thus been shown and described with reference tospecific embodiments, it should be noted that the invention is in no waylimited to the details of the described arrangements; changes andmodifications may be made without departing from the scope of theappended claims.

1. An inverter generator having a generator unit that is driven by aninternal combustion engine and generates alternating current, aconverter that is connected to the generator unit and converts thealternating current to direct current, an inverter that is connected tothe converter and converts the direct current to alternating currentwith switching elements to supply to an electrical load, an inverterdriver that drives the switching elements with a PWM signal generatedusing a reference sine wave of a desired output voltage waveform and acarrier at every control cycle and makes the alternating currentconverted in the inverter to the alternating current of a predeterminedfrequency, comprising: a current detector that detects the alternatingcurrent supplied to the electrical load; a direct current voltagedetector that detects voltage of the direct current converted by theconverter; an alternating current voltage detector that detects voltageof the alternating current supplied by the inverter; an output voltagecorrector that corrects the detected voltage of the alternating currentas a predetermined value based on a coefficient set based on thedetected voltage of the direct current, when the detected alternatingcurrent is greater than a threshold value; and a PWM signal correctorthat corrects the PWM signal by the predetermined value at the controlcycle, such that the detected alternating current becomes less than thethreshold value.
 2. The inverter generator according to claim 1, whereinthe coefficient is set based on the detected voltages at differentcontrol cycles.
 3. The inverter generator according to claim 2, whereinthe coefficient is set based on a ratio of the detected voltages atdifferent control cycles.
 4. The inverter generator according to claim1, wherein the output voltage corrector corrects the detected voltage ofthe alternating current as the predetermined value when a power factoris equal to or greater than a prescribed value.
 5. The invertergenerator according to claim 1, wherein the current detector detects thealternating current as an effective value based on an detected valueobtained by a current sensor.
 6. A method of controlling an invertergenerator having a generator unit that is driven by an internalcombustion engine and generates alternating current, a converter that isconnected to the generator unit and converts the alternating current todirect current, an inverter that is connected to the converter andconverts the direct current to alternating current with switchingelements to supply to an electrical load, an inverter driver that drivesthe switching elements with a PWM signal generated using a referencesine wave of a desired output voltage waveform and a carrier at everycontrol cycle and makes the alternating current converted in theinverter to the alternating current of a predetermined frequency,comprising the steps of: detecting the alternating current supplied tothe electrical load; detecting voltage of the direct current convertedby the converter; detecting voltage of the alternating current suppliedby the inverter; correcting the detected voltage of the alternatingcurrent as a predetermined value based on a coefficient set based on thedetected voltage of the direct current, when the detected alternatingcurrent is greater than a threshold value; and correcting the PWM signalby the predetermined value at the control cycle, such that the detectedalternating current becomes less than the threshold value.
 7. The methodaccording to claim 6, wherein the coefficient is set based on thedetected voltages at different control cycles.
 8. The method accordingto claim 7, wherein the coefficient is set based on a ratio of thedetected voltages at different control cycles.
 9. The method accordingto claim 6, wherein the step of output voltage correction corrects thedetected voltage of the alternating current as the predetermined valuewhen a power factor is equal to or greater than a prescribed value. 10.The method according to claim 6, wherein the step of current detectiondetects the alternating current as an effective value based on andetected value obtained by a current sensor.